Remember the number of lines scrolled into the history (versus cleared

into the history) and when resizing only use scrolled lines and not
cleared lines (which are probably not intended to reappear). From
Chaoren Lin.

tmux.h

@@ -691,6 +691,7 @@ struct grid {
 	u_int			 sx;
 	u_int			 sy;
 
+	u_int			 hscrolled;
 	u_int			 hsize;
 	u_int			 hlimit;